In the circuit below, I want to calculate the gain, $$\frac{I_{out}}{I_{in}}$$
as well as the input and output resistances. I wrote the node equations for the two nodes:
$$V_x (g_{ds}+G_D)-V_{gs}g_{ds}=-gV_{gs}$$
$$V_{gs}g_{ds}-V_x g_{ds}=gV_{gs}+I_{in}$$
The first one show that the two node voltages are proportional,
$$V_x = V_{gs} \left( \frac{g_{ds}-g}{g_{ds}+G_D} \right)$$
and the second one allows me to solve for the current:
$$I_{in}=V_x G_D$$
But looking at the resistor through which I_out flows, I have
$$I_{out}=V_x G_D$$
And therefore the gain is just 1.
Now to find the input resistance, I want to find the ratio $$\frac{V_{gs}}{I_{in}}$$
Expressing one voltage as a multiple of the other, I end up with $$R_{in}= R_D \frac{g_{ds}+G_D}{g_{ds}-g}$$
which is not the right answer (it should be R_D || r_ds). For the output resistance, am I supposed to disconnect the input current source and add one to the output node (that's the tiny bit thats protruding in the upper right corner)? That gives me R_D as the output resistance, which is consistent with the answer I have (the problem is from a past exam).
simulate this circuit – Schematic created using CircuitLab
Best Answer
I don't see how you get Iin = VxGd from the 2nd equation. I agree with you that the two node voltages are proportional from the 1st eqn, but when I solve for Iin in the 2nd eqn (eliminating Vx via your first equation) I get a term -gds(gds-g)/(gds+GD), a partial current, that must be flowing through the independent Vgs source. Applying KCL at the botttom node, then seems to contradict your later statement that Iin = Iout.